<!-- /*
 * @Author: RDSunday 
 * @Date: 2020-03-21 15:17:01 
 * @Last Modified by:   RDSunday 
 * @Last Modified time: 2020-03-21 15:17:01 
 */ -->
<!-- 全屏幕导航从上到下动画 -->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            margin: 0;
            font-family: sans-serif;
        }
        .overlay {
            height: 0%;
            width: 100%;
            position: fixed;
            z-index: 1;
            top: 0;
            left: 0;
            /* background-color: rgb(0,0,0); */
            background-color: rgba(0,0,0,0.9);
            overflow-y: hidden;
            /* 过渡属性 */
            transition: height 0.5s;
        }
        .overlay-content {
            position: relative;
            top: 25%;
            width: 100%;
            text-align: center;
            margin-top: 30px;
        }
        .overlay a {
            padding: 8px;
            text-decoration: none;
            font-size: 36px;
            color: #818181;
            display: block;
            transition: height 0.3s;
        }
        .overlay a:hover, .overlay a:focus {
            color: #f1f1f1;

        }    
        .overlay .closebtn {
            position: absolute;
            top: 20px;
            right: 45px;
            font-size: 60px;
        }  
        @media screen and (max-height:450px) { 
            .overlay  {overflow-y: auto;}
            .overlay a {font-size: 20px}
            .overlay .closebtn {
                font-size: 40px;
                top: 15px;
                right: 35px;
            }
            
        }  

    </style>
</head>
<body>
    <div id="myNav" class="overlay">
        <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>
        <div class="overlay-content">
            <a href="#">About</a>
            <a href="#">Services</a>
            <a href="#">Clients</a>
            <a href="#">Contact</a>
        </div>
    </div>
    <h2>全屏导航-从上到下动画</h2>
    <p>点击一下菜单图标打开导航菜单</p>
    
    <span style="font-size: 30px; cursor: pointer;" onclick="openNav()">&#9776;打开</span>
    <script>
        function openNav() {
            document.getElementById("myNav").style.height = "100%";
        }
        function closeNav() {
            document.getElementById("myNav").style.height = "0%"
        }
    </script>
</body>
</html>